The present invention relates to radar level transmitters used in the process control industry to measure levels of materials in storage vessels, such as tanks. More particularly, the present invention relates to a threshold level calculation for establishing threshold levels that can be used by a radar level transmitter to identify material interfaces which are used to calculate material levels.
Radar level transmitters are used in the process control industry to measure levels of materials contained in a tank or vessel by transmitting a microwave pulse into the tank using a radar antenna, receiving a signal relating to reflections of the transmitted microwave pulse, and detecting material interfaces formed by the materials using the signal. Radar level transmitters are also generally adapted to transmit level information relating to the material interfaces to a distant control system.
The materials in the tank could be in a gas, solid, or liquid state. The microwave pulse reflects off the contents of the tank and a return profile of the tank is generated as a signal or waveform. The waveform represents the amplitude of the reflections of the microwave pulses that are received by the radar level transmitter as a function of time. Peaks in the waveform represent received wave pulses corresponding to portions of the microwave pulse that were reflected off impedance discontinuities within the tank. These discontinuities can include various material interfaces such as an antenna-gas interface, a gas-liquid interface, a gas-solid interface, a liquid-liquid interface, such as a layer of oil on water, a liquid-solid interface, a solid-solid interface, and other types of material interfaces. It is desirable to measure the location of these interfaces in order to determine the quantities of the various types of materials in the tank.
The location or levels of these material interfaces can be established using common Time Domain Reflectometry (TDR) principles once the corresponding time locations of the received wave pulses or peaks in the waveform are established relative to a reference time location. Detection of the time location of the received wave pulses generally includes analyzing the waveform for peaks, which exceed a predetermined threshold value. If the tank includes more than one material, multiple threshold values, each relating to a specific material interface, can be used to detect the levels of the various materials.
There is an ongoing need for improved radar level transmitters. Currently, the threshold values are empirically set by an operator of the radar level transmitter. In addition to being time-consuming and requiring a trained operator, this method can lead to inaccurate threshold value settings, which can result in detection errors and erroneous level measurement. Additionally, the amplitudes of the received wave pulses generally have a dependence on several parameters relating to, for example, the properties of the materials contained in the tank, the tank size, the properties of the radar antenna, and temperature. Thus, the threshold values may need to be adjusted each time one of the parameters affecting the amplitudes of the received wave pulses changes, to prevent erroneous measurements. Automation of the setting of the threshold values could save money by increasing the accuracy of the threshold values and reducing the need for trained personnel.